Diploma thesis in Physics submitted by Florian Freundt born in ...

3.2. Sampling methods 3 Sampling sites and methods
sampling tube. Sampling was interrupted and could not be continued at this depth until the
end of September 2010 (Sample ID C04). Additionally, several extractions of Site C samples
within the mass spectrometer failed due to mechanical damage to the sample sealing and led
to an even more incomplete record. Sporadic sampling during winter and limited access to the
mass spectrometer led to the site finally being omitted from sampling. Furthermore, an O2 and
CO2 measurement in May 2011 proved this site to suffer from atmospheric air contamination in
all three depths as well.
3.2 Sampling methods
Gas samples were extracted from the sites by attaching a Durridge RAD7 Radon Detector as
a pump to the buried sampling tubes via the connector tube inside the equipment boxes. The
perforated tips of the sampling tubes allow for soil air to be sampled from a screen of 10 cm height,
however the actual ground layer being sampled was probably larger. The RAD7 -pump has a
nominal maximum pump rate of 1 l/min, measurements of the pump rate resulted in an actual
maximum pump rate of only 0.45 l/min though [T. Reichel, personal note]. Pressures during
sample pumping were seldom more than 10 mbar below local atmospheric pressure, indicating
low resistance allowing the pump to reach its actual maximum pump rate. Each sampling depth
was pumped for 30 minutes before the actual noble gas sample was taken. The samples A17
to A23 were taken using the Geotech BM2000 Biogas Monitor pump instead of the RAD7 ’s,
omitting accompanying Radon measurements but leading to reduced pump times per sample of
approximately 2 min instead of 30 min 1 . The air within the largest occurring dead space (the 6 m
sampling tube at Site A) is moved away within the first 1.5 minutes of pumping at a pump rate
of 0.5 l/min. As Figure C.6 shows, O2 and CO2 measurements reached stable reading within the
first 60 seconds of pumping with the BM2000. Scheffer and Schachtschabel [2010] gives a range
of 65 – 35 % for pore space fraction of clay soils, assuming a worst case of 35 % leads to an upper
estimate of evacuated soil volume (at 0.5 l/min pump rate and 30 min of pumping) of 0.043 m 3
per sampling depth. Assuming ideally homogenous soil structure leading to an isotropic, circular
air inflow, this would correspond to an sampling radius of 22 cm.
Therefore the depth accuracies for noble gas, O2 and CO2 measurements are given as ±0.2 m
assuming intact sealing, while the depth uncertainties for the temperature data should not
exceed ±0.1 m.
3.2.1 Noble gas samples
The soil air was pumped through a 6x1 mm tube made of deoxidized copper (Wieland cuprofrio R○
Cu-DHP), see Figure C.4. After 30 minutes of pumping, the pump-ward end of the copper tube
was squeezed shut airtight [Wieser, 2006] using a pneumatic plier. Pressure was measured
inline after closing the pump-ward end of the copper tube: once the connection to the pump
1 Pump rates for the BM2000 were specified by the manufacturer as approximately 0.5 l/min. Comparison of
the achieved pumping pressures of the RAD7 and the BM2000 during sampling led to the conclusion that the
BM2000 ’s pump rate is likely higher than specified and lies somewhere in the range of 0.5 to 1.0 l/min.
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